Minimizing air in hydraulic systems is advantageous to maintain optimum performance in the system. Hydraulic fluids typically used in hydraulic systems are capable of holding up to 12% of air by volume at standard atmospheric conditions. The air in the hydraulic systems exists in two forms, as free air and as dissolved air. Dissolved air, as a percent of total fluid volume, is proportional to the pressure maintained on the fluid. When pressure in the hydraulic systems is reduced, for example below its saturation pressure, the dissolved air is released from the hydraulic fluid as free air. Free air is problematic in hydraulic systems because free air can decrease system performance by reducing the overall stiffness of the hydraulic system and can lead to cavitation.
To reduce the amount of free air in the hydraulic fluid, the hydraulic fluid can be retained in a reservoir allowing the free air to rise and escape to atmosphere. To reduce the amount of dissolved air in the hydraulic fluid, a system can be depressurized and the dissolved air manually bled out. Alternatively, the hydraulic fluid can be run through an unpressurized, vented reservoir, that may be located in a service cart for aircraft applications.